A "designer material" derived from plastic could help
protect astronauts on their way to Mars.

by Patrick L Barry

After reading this
article, you might never look at trash bags the same way again.

We all use plastic
trash bags; they're so common that we hardly give them a second
thought. So who would have guessed that a lowly trash bag might
hold the key to sending humans to Mars?

Most household trash
bags are made of a polymer called polyethylene. Variants of that
molecule turn out to be excellent at shielding the most dangerous
forms of space radiation. Scientists have long known this. The trouble
has been trying to build a spaceship out of the flimsy stuff.

But now NASA scientists
have invented a groundbreaking, polyethylene-based material called
RXF1 that's even stronger and lighter than aluminium."This
new material is a first in the sense that it combines superior structural
properties with superior shielding properties," says Nasser
Barghouty, Project Scientist for NASA's Space Radiation Shielding
Project at the Marshall Space Flight Center.

Humans
set off on a journey to Mars, an artist's concept.

To Mars in a plastic
spaceship? As daft as it may sound, it could be the safest way to
go.

Less is more

Protecting astronauts
from deep-space radiation is a major unsolved problem. Consider
a manned mission to Mars: The round-trip could last as long as 30
months, and would require leaving the protective bubble of Earth's
magnetic field. Some scientists believe that materials such as aluminium,
which provide adequate shielding in Earth orbit or for short trips
to the Moon, would be inadequate for the trip to Mars.

Barghouty is one of
the skeptics: "Going to Mars now with an aluminium spaceship
is undoable," he believes.

Plastic is an appealing
alternative: Compared to aluminium, polyethylene is 50% better at
shielding solar flares and 15% better for cosmic rays.

The advantage of plastic-like
materials is that they produce far less "secondary radiation"
than heavier materials like aluminium or lead. Secondary radiation
comes from the shielding material itself. When particles of space
radiation smash into atoms within the shield, they trigger tiny
nuclear reactions. Those reactions produce a shower of nuclear byproducts
- neutrons and other particles - that enter the spacecraft. It's
a bit like trying to protect yourself from a flying bowling ball
by erecting a wall of pins. You avoid the ball but get pelted by
pins. "Secondaries" can be worse for astronauts' health
than the original space radiation!

Ironically, heavier
elements like lead, which people often assume to be the best radiation
shielding, produce much more secondary radiation than lighter elements
like carbon and hydrogen. That's why polyethylene makes good shielding:
it is composed entirely of lightweight carbon and hydrogen atoms,
which minimizes secondaries.

These lighter elements
can't completely stop space radiation. But they can fragment the
incoming radiation particles, greatly reducing the harmful effects.
Imagine hiding behind a chain-link fence to protect yourself in
a snowball fight: You'll still get some snow on you as tiny bits
of snowball burst through the fence, but you won't feel the sting
of a direct hit from a hard-packed whopper. Polyethylene is like
that chain link fence.

"That's what we
can do. Fragmenting - without producing a lot of secondary radiation
- is actually where the battle is won or lost," Barghouty
says.

Made to order

Despite their shielding
power, ordinary trash bags obviously won't do for building a spaceship.
So Barghouty and his colleagues have been trying to beef-up polyethylene
for aerospace work.

That's how Shielding
Project researcher Raj Kaul, working together with Barghouty, came
to invent RXF1. RXF1 is remarkably strong and light: it has 3 times
the tensile strength of aluminium, yet is 2.6 times lighter - impressive
even by aerospace standards.

"Since it is a
ballistic shield, it also deflects micrometeorites," says Kaul,
who had previously worked with similar materials in developing helicopter
armor. "Since it's a fabric, it can be draped around molds
and shaped into specific spacecraft components." And because
it's derived from polyethylene, it's an excellent radiation shield
as well.

Ethylene,
the building block of polyethylene, is rich in hydrogen
and carbon.

The specifics of how
RXF1 is made are secret because a patent on the material is pending.

Strength is only one
of the traits that the walls of a spaceship must have, Barghouty
notes. Flammability and temperature tolerance are also important:
It doesn't matter how strong a spaceship's walls are if they melt
in direct sunlight or catch fire easily. Pure polyethylene is very
flammable. More work is needed to customize RXF1 even further to
make it flame and temperature resistant as well, Barghouty says.

The Bottom
Line

The big question, of
course, is the bottom line: Can RXF1 carry humans safely to Mars?
At this point, no one knows for sure.

Some "galactic
cosmic rays are so energetic that no reasonable amount of shielding
can stop them," cautions Frank Cucinotta, NASA's Chief Radiation
Health Officer. "All materials have this problem, including
polyethylene."

Cucinotta and colleagues
have done computer simulations to compare the cancer risk of going
to Mars in an aluminium ship vs. a polyethylene ship. Surprisingly,
"there was no significant difference," he says. This conclusion
depends on a biological model which estimates how human tissue is
affected by space radiation - and therein lies the rub. After decades
of space flight, scientists still don't fully understand how the
human body reacts to cosmic rays. If their model is correct, however,
there could be little practical benefit to the extra shielding polyethylene
provides. This is a matter of ongoing research.

Because of the many
uncertainties, dose limits for astronauts on a Mars mission have
not been set, notes Barghouty. But assuming that those dose limits
are similar to limits set for Shuttle and Space Station flights,
he believes RXF1 could hypothetically provide adequate shielding
for a 30 month mission to Mars.

Today, to the dump.
Tomorrow, to the stars? Polyethylene might take you farther than
you ever imagined.